Sera from tumor bearing animals can specifically inhibit the in vitro cytotoxic effect of lymphocytes recovered from specifically tumor-immune animals. The effect has been attributed to blocking factors which are hypothesized to exist. The purpose of this proposal is to investigate a duel infection mode system for the study of viral induced blocking factors and tumor enhancement. Lactic dehydrogenase virus (LDV) infection in mice mediates serum blocking factors and enhances murine sarcoma virus (MSV-M) tumor progression. The detection, isolation and quantitation of LDV and MSV-M antigen, antibody, and immune complexes in each system will be carried out as well as in dual infected system. These data will be correlated with the in vivo response of tumor enhancement and the in vitro response of blocking factors in sera to lymphocyte blastogenesis and lymphocyte cytotoxicity. This model provides a definable system for a definitive assessment of the role of viral induced immune complexes and tumor enhancement. Furthermore, the model provides a comparison of in vitro blocking of cell mediated immunity and in vivo tumor enhancement.